# Solar plants and energy conservation

I'd like to apologise if this question is stupid or if it was asked ten thousand times - I haven't found an answer to it. Quite possibly due to mediocre English or abominable knowledge of physics.

So, let's say that we're able to convert 100% of energy that falls on photovoltaic cell to electricity. This means that NO energy is lost. Now assume that all countries on the planet use these cells. Won't it increase temperature on the Earth? Not that I'm really concerned about it, my guess is that even if we'd cover million square kilometers of Earth with them this change won't be very big (if it would happen), but maybe I'm wrong? Would Earth temperature rise? If yes, why? If not, why? How much?

• Well, if we were actually using that energy then yeah that would be bad. But I'm pretty sure solar cell saturate when there isn't a current draw which means they'd fall way below the 100% efficiency. – Brandon Enright Jan 27 '15 at 21:59
• Hah, solar cell saturation. After reading your answer, I quickly searched internet for some information and I read this excellent article on wikipedia about theory of solar cells. Unfortunately I understand nothing :) But thanks for a pointer, I'll read some more. – Jędrzej Dudkiewicz Jan 27 '15 at 22:13

Your question is imprecise but I think it can be reduced to "What would be Earth's surface temperature if it would be a blackbody?", because captured energy would be used and radiated as IR.

As you can find on Wikipedia effective temperature of the planet depends on $(1-a)^{0.25}$, where $a$ is albedo ("whiteness") of the planet. Earth's albedo is about 1/3 and in the case of blackbody it's 0. We can estimate that blackbody Earth would have effective temperature $1/(1-1/3)^{0.25}=1.1$ times - from 252K to 277K (4°C) - it's similar to value 5.3°C which can be found on Wikipedia article about greenhouse effect, which is raising Earth's temperature by over 30K. If this greenhouse effect wouldn't change temperature would rise from over ten to about 40°C.

One could think that incredible usage of solar energy would reduce greenhouse effect, but covering the world with black panels would kill all trees, froze ocean and who knows what else, so maybe it's better to cut this speculation here.

• Thanks a lot for a great answer. It seems that in reality I'm most interested in the part that would follow hadn't you cut your speculations here :) – Jędrzej Dudkiewicz Jan 28 '15 at 7:28

Running the risk of not understanding your question entirely...

... but solar energy is just converted in electric energy in this scheme. Due to the idealized situation of no energy loss, nothing is in fact converted into heat, so I don't see any reason at all why the temperature would rise. If anything, it would drop.

• I assume that it would drop because it won't be used to increase temperature of a surface that is affected? But this means that you also assume that we have batteries that are able to store nearly infinite amounts of energy. What will happen, if we had solar plants with, let's say, 80% efficiency? This energy must be used somehow, would it mean that we'd have to radiate it more effectively into space? Would Earth change into Planet Las Vegas? – Jędrzej Dudkiewicz Jan 27 '15 at 22:10
• Anything is conceivable, since this is a very artificial situation. As long as the energy is stored - in batteries for instance - you are right: then the temperature would drop. We are assuming earth would be covered in solar cells here - batteries to store that energy are not that farfetched, then. If the energy is used in such a way that it is effectively converted into heat, then the Earth could in fact heat up. So it really depends on what you do with the energy you receive. Bottom line: the action of collecting the solar energy does not change the temperature. – user1991 Jan 27 '15 at 22:23

The earth is going to receive an influx of energy (heat) from the sun regardless of whether or not we capture it as electric energy.

If we don't capture it as electricity, that energy eventually radiates into space.

If we do capture it, once that electricity is used, heat will be released, and that heat will eventually radiate into space.

Thanks, space.

• My understanding is that eventually we'll all be dead, so it doesn't matter. But seriously, let's say that 70% of energy that hits one square meter is immediately reflected (well, photons are?) back into space. Now, if we convert it all to energy and use (so heat is generated), won't this energy take a little longer to radiate into space? – Jędrzej Dudkiewicz Jan 27 '15 at 22:05
• Yes, but the rate at which energy radiates into space is proportional to the amount of energy. So whenever the earth warms up by a bit too much, it's quick to get rid of that extra energy. Global warming is a different story because it messes with the earth's ability to radiate said energy. – Señor O Jan 27 '15 at 22:10
• So should I understand that even if energy that is received by Earth is increased, I don't know, threefold, temperature on Earth won't increase? My understanding is that ability to radiate this energy is somehow related to the area that radiates - well, not only "somehow" but directly? So would "we" be really able to radiate this whole energy into space? – Jędrzej Dudkiewicz Jan 27 '15 at 22:16
• The rate of radiation of that energy into space is related to a few things - surface area of the earth is one thing, temperature of the earth is another. "We" aren't the ones radiating all the energy into space, every substance on the earth does it in some form or another, naturally. – Señor O Jan 27 '15 at 22:52
• Ok, I understand. By "we" I meant "everything" on Earth, in the spirit of "we and our globe are one" or something like this. Thanks for an answer. – Jędrzej Dudkiewicz Jan 28 '15 at 7:26